The incidence of spasticity is reported to be 20%-40% in post-stroke survivors. Botulinum neurotoxin (BTX) is considered as the first-line treatment for focal spasticity management. However, BTX injection is costly and may cause side effects or even severe problems in patients. The occurrence and severity of side effects are dependent on the delivered dose of toxin. BTX blocks release of neurotransmitters presynaptically at the neuromuscular junction (NMJ), estimated by innervation zone (IZ) from surface EMG recordings. Studies have demonstrated that increasing the injection distance by 1cm from the IZ of muscle reduced the effect of BTX by 46%. Therefore, it is critical to accurately localize IZs in the 3-dimentional (3D) space of the target muscles to guild the BTX injection for the best clinical effect with the minimum dose. Unfortunately, the IZ locations vary between muscles and individuals, and currently there is no technique available to define 3D IZ distributions in spastic muscles for guiding BTX injections for specific patients. We recently developed a novel 3D IZ imaging (3DIZI) approach by combining the MAI and surface EMG decomposition approaches to image the distribution of IZs in the 3D space of the target muscles from high- density surface EMG recordings. The developed 3DIZI approach has been validated in two healthy male subjects. The Goal of this project is to further evaluate the novel 3DIZI approach in spastic muscles and to examine the feasibility of utilizing the 3DIZI approach to guide BTX injections in treating muscle spasticity. The hypothesis is that the 3D distribution of IZs in spastic muscles can be accurately reconstructed from high- density surface EMG recordings using the 3DIZI approach, and that IZ-guided BTX injections will generate better clinical outcomes with a lower dose compared to standard injection procedure. We will evaluate the performance of the 3DIZI approach in localizing IZs in the 3D space of the spastic muscles in patients with muscle spasticity with simultaneously recorded high-density surface EMG and intramuscular EMG recordings. We will also examine the feasibility of utilizing the 3DIZI approach to guide BTX injections in patients with biceps spasticity by comparing the treatment outcomes of 3DIZI-guided BTX injections against standard BTX injections. This research represents the first effort to image the 3D distribution of innervation zones or propagating muscle activities in target muscles from high-density noninvasive surface EMG recordings. It provides ample opportunities for future basic research and clinical applications, such as the proposed application for guiding BTX injections. The primary impact of this research will lead to 3DIZ-guided BTX injection technique which is innovative and clinically significant. Because BTX is injected to where its needs to be using this technique, the technique is able or expected to significantly reduce the dose, thus the cost of this very expensive treatment with improved clinical outcomes. Moreover, it is expected to minimize possible adverse effects.